Considerable speculation has surrounded the role of central cholinergic neurons in the regulation of arousal and the sleep/waking cycle for nearly three decades. Anatomical and pharmacological data suggest that cholinergic neurons in the laterodorsal tegmental (LDT) and pedunculopontine (PPT) nuclei are parts of an ascending reticular activating system that modulate the behavioral state of an animal via projections to thalamic nuclei. However, the activity patterns of LDT and PPT neurons, which constitute direct evidence on these issues, are virtually unknown. In order to resolve some of these outstanding questions, the specific aim of this proposal will be to electro- physiologically characterize the extracellular single unit activity of putative cholinergic neurons in the LDT and PPT. A strategy is proposed for identifying extracellular recordings from putative cholinergic neurons by using neurophysiological and historical criteria. Since nearly all thalamically projecting neurons in the LDT and PPT are cholinergic, antidromic activation from the thalamus will be used as an initial in vivo criterion identify putative cholinergic neurons . Subsequent histological localization of extracellular recording sites within clusters of neurons stained for positive makers for cholinergic in the LDT and PPT, such as the enzymes choline acetyltransferase (ChAT) or nicotinamide adenine dinucleotide phosphate diaphorase (NADPH-d), will be used as a second criterion. In the first experiment, these criteria will be used to establish the presently unknown extracellular neurophysiological characteristics of putative cholinergic LDT and PPT neurons. In the second experiment, iontophoretic techniques will be used to determine whether putative cholinergic neurons posses somato-dendritic autoreceptors. The results of this study will provide a foundation for the long term objective of relating extracellular single activity of putative cholinergic neurons in the LDT and PPT to changes in behavioral state.